A Finite State Modeling for Adaptive Modulation in Wireless OFDMA Systems

1. A Finite State Modeling for Adaptive Modulation inWireless OFDMA Systems Dania Marabissi, Daniele Tarchi, Federico Genovese, and Romano Fantacci University of Florence, Italy Email: tarchi@lenst.det.unifi.it COST289 4th Workshop - Gothenburg, Sweden

2. Outline • Standard IEEE802.16 • S-OFDMA (Scalable-OFDMA) • Adaptive modulation • Proposed adaptation techniques • Numerical results • Conclusions COST289 4th Workshop - Gothenburg, Sweden

3. Standard IEEE802.16 • WiMAX system, that is based on the IEEE 802.16 specifications, offers broadband wireless accces in a wide are achieving about 50 Km in LoS connections and 10 Km in NLoS connections • The standard aims to have until 70Mbit/s per cell • Almost recently has been finalized the IEEE802.16e version that adds the user mobility functionalities by exploiting the OFDMA technique • This allows to offers mutimedia services in mobility such as Internet, Voice over IP (VoIP) and video streaming COST289 4th Workshop - Gothenburg, Sweden

4. PHY characteristics • It transmits in the 2 – 11GHz band • Recently has been almost defined four operational bands at 2.3, 2.5, 3.5, 5 GHz • It allows to use QPSK, 16QAM and 64QAM, and variable coding rates • Both TDD and FDD (half and full duplex) are allowed • The channel width can varies from 1.75 MHz to 20 MHz and in TDD the frame time can vary from 2 ms to 20 ms • The IEEE 802.16e has improved the OFDMA support COST289 4th Workshop - Gothenburg, Sweden

5. S-OFDMA • The IEEE 802.16e standard has introduced a particolar OFDMA called S-OFDMA (Scalable-OFDMA) • It mainly differs from the classical OFDMA because it can support several channel width by varying the FFT size during the subchannel mapping COST289 4th Workshop - Gothenburg, Sweden

6. Adaptive Modulation • It selects automatically the modulation order for: • minimize the error probability; • maximize the transmitting rate; Channel state adaptativity • The AM (Adaptive Modulation) module manage the adaptivity based one: • channel state information and received power • receiver quality in terms of error rate COST289 4th Workshop - Gothenburg, Sweden

7. TDD Channel information • The proposed system is to be used in the TDD scheme • It aims to estimate the channel behaviour in the uplink and use the estimation for selecting the best modulation order to be used in the downlink COST289 4th Workshop - Gothenburg, Sweden

8. Adaptive management Channel state adaptivity Three state Moore machine COST289 4th Workshop - Gothenburg, Sweden

9. Maximum Throughput Technique • Throughput: Thr(SNR, α, M) = log2(M)[1-SER(SNR, α, M)] • For each fixed SNR we select two thresholds [αA, αB] by solving: Thr(SNR, αA, M=16) = Thr(SNR, αA, M= 4) Thr(SNR, αB, M=64) = Thr(SNR, αB, M=16) 64QAM 16QAM QPSK COST289 4th Workshop - Gothenburg, Sweden

10. Minimum SER Technique • For each fixed SNR at the BS two thresholds are selected [αA, αB]: SER(SNR, αA, M=16) = SER(SNR, M=4) SER(SNR, αB, M=64) = SER(SNR, M=4) where, supposing, we have: SER(SNR, M=4) COST289 4th Workshop - Gothenburg, Sweden

11. Target SER Technique • We can define a target SER value (TSER) based on the Quality of Service requested • For each SNR we search two thresholds [αA, αB]: SER(SNR, αA, M=16) = TSER SER(SNR, αB, M=64) = TSER TSER = constant  SNR Example with SNR=15dB COST289 4th Workshop - Gothenburg, Sweden

12. Decision thresholds COST289 4th Workshop - Gothenburg, Sweden

13. System Parameters • Channel model ITU-R M.1225 (vehicular A) with 6 Rayleigh distributed main paths and Jake Doppler • Bandwidth equal to 10MHz • NFFT = 1024 carriers • Carrier at 3.5 GHz • Maximum speed equal to 125Km/h • Frame duration TDD: • TF = 8ms; • TDL=TUL=3,995ms; • (TTG=RTG=5μs) • OFDMA symbols in each frame: • 40 in DL + 39 in UL • Distributed subcarriers allocations COST289 4th Workshop - Gothenburg, Sweden

14. Minimum SER and Maximum Throughput Maximum throughput allows higher rate but higher SER For SNR=8 dB 16QAM is the most used modulation order 2dB For SNR=16 dB 16QAM is the most used modulation order For SNR=16 dB 64QAM is the most used modulation order COST289 4th Workshop - Gothenburg, Sweden

15. Target SER TSER is imposed based on the requested QoS level COST289 4th Workshop - Gothenburg, Sweden

16. Conclusions • IEEE 802.16e standard has been designed to perform a broadband wireless access by allowing multimedia services to mobile devices; this can be done thanks to the high flexibility in several parts of the system (like modulation, coding, subcarrier allocation). • We propose a state model for adapt the modulation order to the channel behavior; three different techniques has been presented, that differs from the threshold calculation • The Maximum throughput method is foreseen for real-time applications where the throughput has more importance the error rate. • The Minimum SER aims to achieve the best performance in terms of error probability for a certain SNR • The Target SER aims to respect a target in terms of error probability in order to be considered to be used in specific applications. • What we are working on: • Error rate as a performance indication; • Extension of the model for considering variable coding rate jointly with modulation; • Joint modulation/coding adaptation with subcarrier allocation, also by taking into account the upper layers queue size (opportunistic scheduling techniques). COST289 4th Workshop - Gothenburg, Sweden